I. Field of the Invention
The present invention relates to an electromagnetic flowmeter for electrically measuring a flow rate of a fluid flowing through a measuring pipe.
II. Description of the Prior Art
In general, an electromagnetic flowmeter serves to measure a flow of a conductive fluid flowing through a measuring pipe by converting the flow rate into an electric signal using Faraday's electromagnetic induction phenomenon. A conventional electromagnetic flowmeter is constructed as follows. The electromagnetic flowmeter of this type has a measuring pipe formed midway along a main pipe through which a fluid to be measured flows. The measuring pipe is coupled to the main pipe through flanges thereof. An insulating lining which is made of a fluorine-contained polymer such as Teflon (trademark) is formed inside (or lined inside) the measuring pipe. A pair of saddle-shaped exciting coils are wound around at two sides of the measuring pipe so as to oppose each other. These exciting coils are held by a core and a housing. When the exciting coils are excited, a magnetic field is generated such that its magnetic flux crosses perpendicularly the flow of the fluid flowing through the measuring pipe. A pair of electrodes are arranged to be perpendicular to both the flow of the fluid and the magnetic flux and are connected to terminals through lead wires. These electrodes are insulated from the measuring pipe and respectively extend through wall portions of the measuring pipe such that their distal ends are slightly exposed inside the measuring pipe.
When the fluid flows through the measuring pipe, the coils are excited. A magnetic field is generated such that its magnetic flux crosses perpendicularly the flow of the fluid. The direction of the fluid flow, the direction of magnetic flux, and the direction of the arrangement of electrodes are perpendicular to each other. An electromotive force which is proportional to the average velocity of the fluid flow is generated across the electrodes in accordance with Faraday's law. The electromotive force is supplied to and amplified by an amplifier to generate a signal indicating the flow rate, thereby allowing control of the flow rate to be constant.
FIG. 1 is a sectional view showing the coupling condition between a measuring pipe 1 of a conventional electromagnetic flowmeter and a main pipe 3. A flange 2a of an insulating lining 2 which lines the measuring pipe 1 contacts the outer surface of a flange 1a of the measuring pipe 1. The flange 2a and a flange 3a of the main pipe 3, respectively, are coupled through a pair of gaskets 4 and a ground ring 5 interposed therebetween. The flange 1a is connected to the ground ring 5 via a cord 6. The ground ring 5 serves to hold the potential of the fluid flowing through the main pipe 3 and the measuring pipe 1 to be the same as that of the electromagnetic flowmeter as a whole. This potential corresponds to a reference potential of a detected current measured by the flowmeter. On the other hand, a pair of electrodes are insulated from each other between the measuring pipe 1 and the fluid. It should be noted that the measuring pipe 1 is not insulated from the flowmeter body. For this reason, the ground ring 5 must be connected to the flange 1a through the cord 6.
As may be apparent from the above description, the conventional electromagnetic flowmeter requires the ground ring 5 and the two gaskets 4, resulting in cumbersome, time-consuming piping and wiring procedures. Furthermore, the fluid tends to leak from the coupled portion, resulting in inconvenience.